The present invention relates to information and communications systems and particularly to those systems wherein it is desirable that a given narrow-band component of a composite signal be preserved substantially intact when other information in the composite signal is amplitude limited. The embodiment described herein is adapted, for example, for use in FM stereo broadcasting.
In FM stereo radio broadcast practice as prescribed by the Federal Communications Commission (FCC), information derived by subtracting left and right audio channels (L-R) amplitude modulates a 38 kHz stereo subcarrier. This subcarrier is then substantially removed leaving two sideband signals. The two sideband signals and the sum of the left and right audio channels (L+R) are added to a 19 kHz stereo pilot tone synchronizing signal. The resultant composite signal is used to frequency modulate a radio station's government designated carrier frequency.
To prevent interference between the designated broadcast frequencies, permissible carrier frequency modulation is currently restricted by the FCC to a peak deviation of not more than 75 kHz. The amplitude of the composite signal modulating the carrier must be limited to avoid exceeding this modulation constraint. Further, in commercial FM broadcasting, maximum apparent strength for the audio signal is commonly considered desirable, which increases the incidence of signal peaks likely to produce excessive modulation and carrier deviation.
Apparatus for limiting the amplitude of an audio input is well known in the broadcasting and recording arts. Common amplitude limiting techniques, however, produce large amounts of high frequency information, i.e., distortion products, which themselves are subject to distortion in subsequent processing. Thus, in the case of FM stereo broadcasting, it is desirable to limit amplitude after the composite signal has been generated.
The STA-MAX wide-band modulation controller manufactured by Automated Broadcast Controls is a limiter that acts on the composite signal. In varying the gain applied to the composite signal to restrict its amplitude, however, this limiter causes amplitude modulation of the stereo pilot tone. This modulation of the station's pilot causes degradation of stereo reception for radio receivers in which the modulated pilot tone only intermittently exceeds the receivers' detection threshold for that pilot tone. The minimum amplitude of the station's amplitude-modulated stereo pilot may also be inadequate to meet the minimum signal quality standards required by the government for stereo broadcasts.